Organic matter possesses a high surface area and contains an abundance of negative charges.
These negative charges contribute to the nutrient retention capacity of soils by attracting positively charged ions (cations) in the soil such as calcium, magnesium, potassium, ammonium and others. These would otherwise leach and get lost in the soil profile. It should be noted that negatively charged ions (anions) such as nitrate and sulphate are not held by negative charges or organic matter and are therefore subject to leaching loss.
Organic matter acts as a chemical buffer by resisting rapid changes in pH. On the positive side, this mechanism delays soil acidification, particularly in soils subjected to long-term fertilisation by urea and ammonium-containing fertilisers. On the negative side, one may need to apply larger quantities of liming material in order to raise the pH of an already acidic soil to a desirable level.
Organic matter complexes cations, particularly micronutrient cations such as iron and zinc, through a process known as chelation. The formation of iron-organic complexes makes insoluble iron more available for plant use.
See other functions of organic matter